Monday, 15 December 2014

Brief History of Astronomical Photography

Brief History of Astronomical Photography

Niépce and Daguerre

In 1824 Joseph "Nicéphore" Niépce (at left; 1765-1833) created the first semi-permanent images using glass plates coated with bitumen dissolved in lavender oil. In the early 1830's Niépce's partner, Louis Jacques Mandé Daguerre (at right; 1787-1851) accidently discovered a method for creating a permanent image on a photographic plate, which was simply a thin film of polished silver on a copper base, sensitized by exposing the silver face to iodine vapors to form a thin yellow layer of silver iodide on the surface of the silver.
After a photograph was taken on the plate, it was developed by exposing the plate to a current of mercury vapor heated to a temperature of 75° Celsius. The vapor would adhere to the part of the plate which had been exposed to the light. The plate was then fixed by immersing it into sodium thiosulfate, which was used to dissolve the unused silver iodide, and finally rinsed in hot water to remove any remaining chemicals. The importance that photography could have in the field of astronomy was immediately realized. It would allow an accurate and easy recording of brightness, positions, spectra, and physical aspects of celestial bodies. However, these early photographic plates were not sensitive enough to image faint objects. The first daguerrotype of the moon was made by American physiologist and chemist John William Draper (at left; 1811-1882) in 1840, involving a full 20 minute exposure. The first star was not recorded until the night of July 16-17, 1850, when William Cranch Bond, the director of Harvard College Observatory, and J. A. Whipple, a photographer associated with Massachusetts General Hospital, took a daguerrotype of Vega. At right is an 1852 daguerrotype of the Moon taken by Whipple.

Wet Collodion Process

Astronomers were not thrilled with the prospect of waiting hours and hours to get an image of a single star or nebula, however. They needed a method to produce better quality images in less time. In 1851, Frederick Scott-Archer (at left; 1813-1857) published an article describing the wet collodion process, although Gustave le Gray (1820-1884) and Robert J. Bingham (1824-1870) earlier had suggested and experimented with the technique. This process produced a plate which had a much higher sensitivity than the early daguerrotypes, but it needed to be used as soon as it was made. Furthermore, the process for producing such plates was much more complicated. Sulfuric acid and potassium nitrate were reacted on a small quantity of cotton to create guncotton (nitrocellulose). This guncotton was then dissolved in alcohol and ether with iodides and bromides of cadmium, potassium, and ammonium. The colloid which was produced was then spread on glass plates and evaporated to leave a thin film of nitrocellulose impregnated with bromides and iodides. When the plates were dry, they were dipped into silver nitrate which was saturated with silver iodide, and this transformed the iodide and bromide into salts of silver. This silver halide coating was then sensitive to light, but the plate had to be used immediately, or else the silver nitrate would crystallize. After the image was taken, the plate was developed in a bath of iron sulfate, acetic acid, and alcohol which turned the exposed silver halide grains into metallic silver. Sodium thiosulfate was used as a fixer to remove the remaining (unexposed) silver halide grains, and the plate was then washed to remove the chemicals. Finally a coat of varnish was applied to protect the image.
Mizar and Alcor were photographed in March 1857 at Harvard College Observatory on wet collodion. The 1874 transit of Venus was also widely photographed on collodion plates as well as daguerrotypes. The collodion plate at right was taken in Japan by Jules Janssen (1824-1907), later director of the Meudon Observatory.

Silver Bromide Dry Emulsions

But again astronomers were inconvenienced by the fact that these wet plates had to be used immediately after they were produced, and although they had a higher sensitivity to light, the extra sensitivity often was not made up for by the extra time and effort it took to have the plates ready to go for the night's observing. The next phase of development, then, was to create a plate which was highly sensitive to light, but which had a dry rather than wet surface, so it did not need to be used immediately. During the decade of the 1870's, there were several dramatic technological breakthroughs in the field of photography.
 In 1871 Richard Leach Maddox (at left; 1816-1902), a physician and photographer, produced the first positive dry emulsion for physical development, using gelatin (a transparent animal protein), and then in 1874, J. Johnston and W. B. Bolton made the first negative emulsion for chemical development. By 1878, Charles Bennett had discovered a method by which he could increase the speed (sensitivity to light) of gelatin-silver bromide emulsions (at right) by aging them at 32°C in a neutral medium. This was a most important development for the field of astronomy, since the universe is filled with very faint objects, and astronomers wanted to be able to photograph them without waiting for days and days to get an image on a photographic plate. In 1879, George Eastman (1854-1932) invented a machine to coat plates with emulsion, so that the plates (at left) could be produced in mass numbers, relatively quickly and cheaply. Utilizing the new silver bromide dry emulsion plates, the first good photographs of Jupiter and Saturn were made in 1879-1886, and of comets in 1881 (Tebbutt's comet). A 51 minute exposure of the Orion Nebula was taken in September 1880 by Henry Draper (at right; 1837-1882), a doctor and prominent amateur scientist (and the son of John William Draper), and two years later he took another lasting 137 minutes which revealed the entire nebula and the faintest stars in it. The study of spectra could also be undertaken with the new plates, since they were so much more sensitive to light than those previously. In 1872, the first spectrum of a star-Vega-was taken by Henry Draper. In 1882 Sir William Huggins (who was the first to show that stellar spectral lines could be identified with terrestrial elements, in 1864) took the first spectrum of a nebula (the Orion Nebula), and in 1899 the first  spectrum of a "spiral nebula" (now known as a spiral galaxy and much more distant than anything else photographed before) was taken, a 7½ hour exposure taken by Julius Scheiner (at left; 1858-1913) with the Große Refractor of the Astrophysical Institute of Potsdam Observatory. The new kind of plates also brought along with it the era of sky surveying, systematically photographing large expanses of sky. The first sky surveys were done at Harvard during the period 1882-1886, each photograph covering a 15°x15° area of the sky and reaching stars as faint 8th magnitude.

Emulsion Grain Size and Color Sensitivity

A close look at any photograph, particularly one which has been blown up, reveals a certain graininess. Because photographic emulsions are made up of particles in suspension, this graininess can not be completely eliminated and so at some level there will always be a loss of detail in taking a photograph. The first emulsions which were developed had grain sizes of about 10 micrometers in diameter. Although this seems tiny relative to most things that we know, such large grains could result in a loss of detail in certain circumstances (excellent seeing and resolution). More recently, finer grain emulsions became available (with typical grain sizes of about 1 micrometer) which can be used in order to exploit excellent observing conditions to produce more images with more fine detail. However, the smaller grain size results in a drastically reduced sensitivity, since the amount of light striking an individual grain has now decreased when compared to larger grained emulsions. Exposure times are significantly longer for fine grained emulsion, and hypersensitization techniques are often employeed (see next section).
Hermann Wilhelm Vogel (at right; 1834-1898), working in Berlin in 1873, accidentally discovered a way to make photographic emulsions sensitive to colors of light other than blue. At the time, green dye was used to soak up reflections off the back side of the glass in a photographic plate. Sometimes this green dye got into the emulsion along the plate edges, and Vogel noticed that the plate in this area was more sensitive to light of a longer wavelength or redder color. This observation was quickly exploited in making new kinds of emulsions which were sensitive at all of the visible colors of light, and by just a year later, Sir William de Wiveleslie Abney (1843-1920) was able to put together an entire optical solar spectrum, from violet to infrared. During the first couple of decades of the twentieth century, C. E. Kenneth Mees (1882-1960) at Eastman-Kodak made outstanding improvements in emulsions and spectral sensitivity. Mees grew particularly interested in the astronomical applications of these new emulsions and so he formed a partnership with several observatories in developing new ways to satisfy their needs, and insisted that Eastman-Kodak provide these plates to astronomers at cost.

Eastman-Kodak and Hypersensitization

During the twentieth century, Eastman-Kodak (George Eastman at right) was the leading producer of new, faster emulsions. One of the major problems with photography of very faint objects, as is often the case in astronomy, is that the emulsions may react with the incoming light, but the emulsions react differently with light which has come in at a quick rate versus light which slowly filters in. For example, if a plate receives, say, 100 photons all at once, it will have no trouble reacting with them, but if the plate receives those same 100 photons over a period of an hour, it will probably not detect the light. And since astronomical light often filters in rather slowly, over a longer period of time, the emulsions do not usually detect it as well. This phenomenon is known as reciprocity failure.   The first person to determine a way to partially overcome this problem was Fox Talbot (at left; 1800-1877) in 1843, who discovered that heating emulsions prior to exposing them increased their efficiency for short exposures. Fifty years later, William Abney and King found that chilling emulsions during the exposure made them more efficient for long exposures. It was not until the mid-twentieth century that scientists at Eastman-Kodak and elsewhere put together true scientific studies of why these different techniques worked and what other techniques might work even better for hypersensitizing the emulsions. I.S. Brown and L.T. Clark in 1940 published results of their tests of water bathing, pre-exposure, ammoniating, mercury-vapor treatment, and high temperature baking for several different emulsions. This study then inspired many astronomers to attempt hypersensitizing their own photographic plates, and soon the American Astronomical Society created a Working Group on Photographic Materials to study the problem.
After years of research, it has been concluded that different methods of hypering plates yield different results. For instance, the method of pre-exposure involves flashing a light on a plate before the actual exposure is taken for the purpose of reducing the total exposure time of the plate. Thus, image specks will form more quickly and be more stable against decay, so subsequent light is absorbed efficiently. Cooling a plate during exposure, as discovered by Talbot, works by increasing the lifetime of the silver atoms liberated (by the incoming photons) from the silver halide crystals. These silver atoms then survive for long enough to aggregate to make developable latent-image specks. Plates also are baked in nitrogen, oxygen, or just air before exposure. The result is a reduction of the reciprocity inefficiencies (the best improvements are seen for the nitrogen bake, with the least seen from baking in air). Another technique involves soaking a plate in nitrogen or hydrogen gas at room temperature. This helps to drive out the oxygen and water present in the gelatin. Emulsions to absorb infrared light have also been developed, but they are much more sensitive to heat and so much more delicate. However, they can also be hypersensitized, in this case by placing them in a high humidity, oxygen-free environment. For example, they are usually hypered in a bath of distilled water, which results in a gain in speed, or else a bath of ammonia or silver nitrate solution, which helps to remove bromide or iodine ions in solution in the gelatin. The removal with a silver niteate solution will dramatically increase the sensitivity of the IR emulsion.

Newer Photographic Techniques

Several techniques to obtain the most information from a photograph have been developed David Malin at the Anglo Australian Observatory.
All photographs suffer from some degree of granulation due to effects in our own atmosphere and also from irregularities in the emulsion itself. A technique for removing these imperfections was invented in the middle part of the twentieth century. If an astronomer can take several images almost simultaneously, each of which presumably would have slightly different granulations, they could then superimpose or "stack" the images and thus remove any irregularities which are not seen in all of the images. This technique is displayed in the series of images of NGC 4672 by David Malin, at right. A method was also developed for detecting very faint and extended objects such as nebulae, which are often not noticed in traditional photographs because they blend into the background light. However, by superimposing the glass photographic negative onto a positive print which was made from light of a different color, astronomers can easily see, for example, blue stars as black spots with white halos around them and red stars as white stars with black halos around them. This contrast more easily allows astronomers to detect nebulae and other faint objects. Additional techniques, such as Photographic Amplification and Unsharp Masking, have allowed some of the lowest surface brightness objects to be discovered, including the giant low surface brightness spiral galaxy Malin.